Viscocanalostomy in Rhesus Monkeys

Abstract

Objective: To examine structural changes and aqueous humor outflow after viscocanalostomy in live normal monkey eyes.Methods: Viscocanalostomy surgery was performed in 1 eye of each of 4 rhesus monkeys. Outflow facility was determined before and after surgery. All eyes were fixed and examined by light and/or electron microscopy 36 or 63 days postoperatively.Results: Schlemm canal was replaced by scar tissue at the surgical site. The juxtacanalicular zone contained homogeneous material, probably high-molecular-weight 1.4% sodium hyaluronate. The sclera external to Schlemm canal was overhydrated, and remains of a scleral lake were present in 1 animal. Multiple defects were present in the endothelial lining of Schlemm canal inner and outer wall. Fine fibrillar material and sheath-derived plaque material partly bridged the defects. Along the inner wall, aggregations of thrombocytes covered some defects in the endothelial lining of the canal. At 90°to 180°from the surgical site, small and fewer breaks in the inner wall were seen. Postsurgery outflow facility (n=2) was approximately 30% higher in the treated eye than in the contralateral control, corrected bilaterally for presurgery baseline. Conclusions:The most likely explanations for the increase in outflow facility in monkeys after viscocanalostomy are focal disruptions of the inner wall endothelium of Schlemm canal and disorganization of the juxtacanalicular zone, resulting in direct communication of juxtacanalicular zone extracellular spaces with the lumen of Schlemm canal. The continuous presence of sodium hyaluronate might prevent repair of these defects by interfering with thrombocyte function.Clinical Relevance: In nonhuman primates, viscocanalostomy appears to decrease outflow resistance through persisting focal disruption of the inner wall endothelium and opening of the juxtacanalicular or cribriform region of the trabecular meshwork, the tissue most affected by pathologic changes in primary open-angle glaucoma in humans.

Phaco-viscocanalostomy versus Phaco-trabeculotomy

“…Regarding VCS, few reports have been published on PEG 25 ; however, the reported morphologic evidence suggests that VCS achieves an IOP reduction in cases of PEG. Focal disruption of the inner wall of the endothelium and the opening of the juxtacanalicular or cribriform region of the trabecular meshwork has been reported, 26,27 and we also observed leakage of viscoelastic material and a blood reflux from Schlemm canal into the anterior chamber using an ophthalmic endoscopes in vivo. 28 These findings are similar to trabeculotomy, thus, VCS would be effective in patients with PEG.…”

Application of Canaloplasty in Glaucoma Gene Therapy: Where Are We?

“…Tamm et al 19 performed viscocanalostomy surgery for the first time in rhesus monkeys. We performed our initial experiments in enucleated rhesus and cynomolgus eyes and our live-animal experiments used both a cynomolgus and rhesus monkey.…”

“…18 Viscocanalostomy has been shown to decrease IOP in patients with POAG and also increase outflow facility in nonhuman primates. 17,19 Canaloplasty uses the same methods to unroof SC, whereafter a flexible microcatheter with a lighted tip is inserted into one ostium and passed circumferentially to dilate and stretch SC until it exits the other ostium. A 9-0 or 10-0 monofilament suture is then tied to the catheter tip and passed through the SC as the catheter is being withdrawn.…”

Comparing Viscocanalostomy with Trabeculectomy

“…Injection of viscoelastic into the canal not only dilates the canal and associated collectors but also disrupts the internal and external walls of Schlemm's canal and adjacent trabecular layers, thus increasing the trabecular outflow facility and making the procedure act as a 'micro-trabeculotomy' (see Figure 5). 2 Aqueous outflow facility is also increased by damage to the inner wall of Schlemm's canal and adjacent trabeculum at the site of surgery, thus enhancing aqueous outflow into the scleral lake. From here, aqueous can leave the eye via three different paths:…”